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Macromolecules 2008, 41, 2990-2993
Stereospecific Ring-Opening Metathesis Polymerization of 3-Methyl-3-phenylcylopropene by Molybdenum Alkylidene Initiators Rojendra Singh and Richard R. Schrock* Department of Chemistry 6-331, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 ReceiVed February 6, 2008 ReVised Manuscript ReceiVed March 19, 2008 Recently we reported the successful ROMP of 3,3-disubstituted cyclopropenes, e.g., 3-methyl-3-phenylcyclopropene (1).1 To our knowledge this is the only report of ROMP of a cyclopropene.2 Most of these polymerizations appear to be living, perhaps largely because the alkylidene in the propagating species resembles a neopentylidene or neophylidene species, which are known to be the most stable terminal alkylidenes in the Mo(NR)(CHR′)(OR′′)2 family toward bimolecular decomposition.3 It was proposed that trans linkages are formed between the repeat units in poly(1) as a consequence of steric hindrance in the molybdabicyclopentane intermediate. If the substituents on C(3) of the cyclopropene are different, as is the case for 1, then tacticity is possible. Below are shown the two regular structures for trans-poly(1). We report here the preparation of highly tactic trans-poly(1) through enantiomorphic site control with initiators that are related to those that have been employed for asymmetric metathesis reactions.4 Figure 1. Initiators that contain relatively electron-withdrawing diolate ligands. Compounds 2a, 2b, and 2c contain racemic biphenolate ligands. All binaphtholate ligands have the R configuration, as drawn.
In Figure 1 are shown several recently reported molybdenum species5 that contain new biphenolate or binaphtholate ligands. Polymerization of 16 using 2a yielded a polymer whose 13C NMR spectrum (Figure 2) is characteristic of a relatively longrange order. In particular, a single sharp resonance is observed for the quaternary carbon atom at 46.05 ppm and for the olefinic carbon atom at 137.23 ppm. An IR spectrum of poly(1) reveals absorbances at 963 and 982 cm-1 of approximately equal intensity that are not present in the spectrum of 1 and that are most consistent with the presence of trans CdC bonds in the polymer7 (see Supporting Information). Therefore, we propose that poly(1) prepared with 2a has a trans and highly tactic (>99%) microstructure and that the 46.05 and 137.23 ppm resonances in the 13C NMR spectrum are characteristic of that structure. The 1H NMR spectrum of this polymer is also sharp and well-resolved. DSC studies (Table 1) revealed a glass transition temperature (Tg) of 105 °C, while the GPC displayed a unimodal peak with a PDI of 1.80. Polymerization in THF or with initiators in which the steric bulk of the imido ligand is decreased (as in 2b or 2c) also led to tactic trans-poly(1), but the PDI did not improve (Table 1). The polymer prepared using 2c revealed a high molecular weight peak (∼40% of the total) in addition to that with Mn ) 12 000. At this stage it is * To whom correspondence should be addressed.
Figure 2. A 125 MHz poly(1).
13C
NMR spectrum (CD2Cl2) of tactic trans-
not clear why the polydispersity indices are relatively high (∼2). Initiator 3a also afforded tactic trans-poly(1) with a relatively low PDI on the basis of 13C NMR and GPC studies. A THFfree version of 3a (3a′) could be prepared by reacting Mo(NAr)(CHCMe2Ph)(2,5-Me2NC4H2)2 (Ar ) 2,6-diisopropylphenyl; 2,5-Me2NC4H2 ) 2,5-dimethylpyrrolide) with 1 equiv of H2[BinaphC6F5] in CH2Cl2.5 The 1H and 13C NMR spectra of poly(1) prepared using 3a′ were identical to those prepared using 3a, although the PDI of the polymer was somewhat higher. So at least in the case of the molybdenum initiator that contains the [BinaphC6F5]2- ligand, the presence of 1 equiv of THF or 2 equiv of 2,5-dimethylpyrrole during polymerization had no
10.1021/ma800280q CCC: $40.75 2008 American Chemical Society Published on Web 04/08/2008
Macromolecules, Vol. 41, No. 9, 2008
Communications to the Editor 2991
Table 1. Characterization Data for Poly(1) Prepared in
Dichloromethane Using Initiators 2-5 initiator solvent
PDIa
Tgb
Mna
yield (%) tacticity (%)c
2a CH2Cl2 1.80 105 12 000 95 99 2a THF 2.17 105 11 000 96 99 2b CH2Cl2 2.07 105 12 000 98 >99 2b THF 2.18 105 12 000 95 >99 2c CH2Cl2 1.87 102(100) 12 000 90 >99 2c THF 2.13 102(100) 9 000 93 >99 3a CH2Cl2 1.18 107(105) 14 000 94 >99 3a′d CH2Cl2 1.93 107 10 000 94 >99 3b CH2Cl2 2.12 f 22 000 10 atactic 3c CH2Cl2 f f f
